1. Field of the Invention
The present invention relates generally to a system and method for retransmitting packet data from a mobile terminal in an asynchronous Code Division Multiple Access (CDMA) mobile communication system servicing an enhanced uplink dedicated transport channel (EUDCH), and in particular, to a system and method for retransmitting packet data from a mobile terminal in a soft handover region.
2. Description of the Related Art
In general, a user equipment (UE) selects its data rate to be below a preset available highest data rate. The highest data rate is provided by a radio network controller (RNC). Thus, a Node B does not participate in controlling the uplink data rate. However, the Node B determines the availability of uplink transmission and the available highest data rate for the EUDCH and transmits the information to the UE based on a scheduling command. The UE then determines its data rate according to the scheduling command. The EUDCH was designed to improve the performance of uplink packet transmission in an asynchronous mobile communication system.
FIG. 1 is a bock diagram illustrating an example of scheduling in a Node B to service the EUDCH in an asynchronous CDMA mobile communication system.
Referring to FIG. 1, a Node B 110 is one of the active Node Bs supporting a packet data service on the EUDCH. UEs 112, 114, 116 and 118 transmit packet data to the Node B 110 on the EUDCH. Reference numerals 122, 124, 126 and 128 denote the EUDCH operating at data rates determined by scheduling in the Node B 110.
Usually, as the data rate of a UE increases/decreases, its transmission power also increases/decreases, respectively. This implies that a signal at a high data rate than the current data rate of the UE greatly influences an Rise Over Thermal (ROT) measurement of the Node B and a signal at a low data rate than the current data rate of the UE slightly influences the ROT measurement of the Node B. That is, as the uplink data rate increases, more uplink radio resources are occupied. The Node B schedules EUDCH packet data by considering the relationship between the data rate and radio resources and a requested data rate.
The Node B 110 determines the availability and the data rate of the EUDCH for each UE using the EUDCH according to a UE-requested data rate and channel condition. This scheduling is done in the manner that a low data rate is assigned to a remote UE and a high data rate to a near UE, while an ROT measurement does not exceed a target ROT. In FIG. 1, the distances between the UEs 112 to 118 and the Node B 110 are different. The UE 116 is nearest to the Node B 110, whereas the UE 112 is furthest away from the Node B 110. As indicated by arrows 122 to 128 having different thicknesses, the UEs 112 to 128 use different transmission powers according to their distances to the Node B 110. The transmission power of the nearest user equipment which is UE 116, is the smallest as indicated by the least thick arrow 126, while that of the user equipment which is the furthest away UE 112, is greatest as indicated by the thickest arrow 122. Therefore, the Node B 110 schedules EUDCH data such that the transmission power is inversely proportional to the data rate in order to achieve the best performance, while the same ROT is maintained and inter-cell interference is reduced. The Node B 110 then assigns the lowest data rate to the UE 112.
FIG. 2 is a call flow diagram illustrating an example of a signaling flow between a Node B and a UE for an EUDCH service in the asynchronous CDMA mobile communication system. The Node B and the UE are assumed to be the Node B 110 and the UE 112 illustrated in FIG. 1.
Referring to FIG. 2, an EUDCH is established between the Node B 110 and the UE 112 in step 201. The EUDCH setup involves transmission/reception of messages on a dedicated transport channel. After the EUDCH setup is completed, the UE 112 transmits to the Node B 110 a data rate and uplink channel condition information which is required for scheduling in step 202. The uplink channel condition information includes information about uplink transmission power and a transmission power margin. In step 203, the Node B 110 estimates a forward channel condition by comparing the uplink transmission power and reception power. If the difference between the transmission power and the reception power is narrow, the Node B 110 assumes that the channel condition is good, and if the difference is wide, the Node B 110 assumes that the channel condition is bad. If the transmission power margin information is received as the uplink channel condition information, the Node B 110 estimates the uplink transmission power by subtracting the transmission power margin from the already-known available maximum transmission power of the UE 112. The Node B 110 determines the available highest data rate for the EUDCH using the estimated channel condition and a data rate requested by the UE. In step 204, the Node B 110 provides the determined highest data rate to the UE 112. Specifically, the Node B 110 determines modulation schemes and the numbers of codes for packet data transmission in the next transmission time interval (TTI) from UEs including the UE 112 to which the EUDCH service is available in step 203. Thus, the Node B 110 assigns the modulation scheme and the number of available codes to the UE 112 in step 204. Scheduling is Node B-dependent. In step 205, the Node B 112 selects its data rate according to the received highest data rate, and also selects a Transport Format and Resource related Information (TFRI) for the EUDCH packet data in order to allows the Node B 110 to prepare for packet data reception. The UE 112 transmits control information containing the TFRI and the data rate to the Node B 110 in step 206. The TFRI information may include information about orthogonal variable spreading factor (OVSF) code, modulation, data size, and HARQ. In step 207, the UE 112 transmits the packet data to the Node B 110 on the EUDCH. The Node B 110 checks errors possibly generated in the received packet data and selects an Acknowledgement (ACK) signal or an Negative ACK (NACK) signal according to the error check result in step 208. In step 209, the Node B 110 transmits the ACK/NACK signal to the UE 112.
FIG. 3 is a block diagram illustrating an example of a soft handover for a UE in the asynchronous CDMA mobile communication system. Referring to FIG. 3, data from a UE 304 in a soft handover region reaches a plurality of active Node Bs 301, 302 and 303 covering the soft handover region. A Node B that has successfully demodulated the received data without errors transmits the demodulated data to an RNC 305. The RNC 305, since it receives the same data from a plurality of Node Bs, achieves a selective diversity gain. This soft handover operation is widely implemented in existing mobile communication systems and also applicable to the EUDCH service.
For application of the soft handover operation to the EUDCH service, the node Bs 301, 302 and 303 receive EUDCH packet data from the UE 304. If they receive the EUDCH packet data without errors, the Node Bs 301, 302 and 303 transmit the received packet data to the RNC 305. If the packet data has errors, the Node Bs 301, 302 and 303 request the UE 304 to retransmit the EUDCH packet data. Since the RNC 305 receives the same data from a plurality of Node Bs, it can ensure a required EUDCH packet data reception performance, minimizing the uplink transmission power of the UE 304.
Hybrid Automatic Retransmission Request (HARQ) is significant to the EUDCH service as it is to High Speed Downlink Packet Access (HSDPA) of 3GPP and 1xEV-DV of 3GPP2. Especially when a UE is in a soft handover region, the importance of HARQ becomes great because it is closely related to the whole system efficiency. The reason for supporting soft handover in the uplink mobile communication system servicing the EUDCH is to service stable uplink data transmission irrespective of the location of the UE in the active Node Bs. Therefore, it is not reasonable to adopt HARQ of the HSDPA system as it is. When a plurality of active Node Bs simultaneously service an EUDCH to a UE, uplink data retransmission by HARQ occurs as illustrated in FIG. 4.
FIG. 4 is a call flow diagram illustrating an example of an uplink packet transmission from a UE in a soft handover region to a plurality of active Node Bs in the saynchronous CDMA mobile communication system supporting the EUDCH. It is assumed that signaling is performed in the same manner as signaling between the UE 304 and the Node Bs 301, 302 and 303 illustrated in FIG. 3.
Referring to FIG. 4, the UE 304, which is placed in a soft handover region, transmits uplink data #1 to the Node Bs 301, 302 and 303 on an EUDCH in steps 421, 431, and 441. The Node Bs 301, 302 and 303 check errors in the received data and transmit ACK/NACK signals to the UE 304. It is assumed herein that the uplink data #1 delivered to the Node Bs 301 and 302 have errors. Thus, the Node Bs 301 and 302 transmit NACKNode B #1 to the UE 304 in steps 453 and 452, while the Node B 303 transmits ACKNode B #1 to the UE 304 in step 451. The Node B 303 demodulates normal data #1 and transmits the demodulated data to the RNC 305. The RNC 305 transmits the received data #1 to a higher-layer network in step 414. At the same time, the UE 304 receives ACKNode B #1 or NACKNode B #1 for the uplink data #1 from the Node Bs 301, 302 and 303. The UE 304 transmits new packet data #2 on an EUDCH to the Node Bs 301, 302 and 303 in response to ACKNode B #1 from the Node B 303 in steps 422, 432 and 442.
As the UE 304 receives different response signals, ACK and NACK from the Node Bs 301, 302 and 303, retransmission by HARQ is not decided with reliability. Moreover, since the RNC 305 guarantees space diversity for the uplink data #1 received in different paths, errors in the uplink data #1 received at the Node Bs 301, 302 and 303 are not corrected.
FIG. 5 is a block diagram illustrating examples of a channel structure for delivering ACK/NACK information from a Node B to a UE in the asynchronous CDMA mobile communication system supporting the EUDCH.
Referring to FIG. 5, each of a plurality of Node Bs demodulates uplink packet data received from the UE and selects ACK/NACK information 510 indicating normal/defective packet reception. Each Node B transmits the ACK/NACK information 510 and downlink information (an Information field) to the UE on a dedicated transport channel supporting the EUDCH service or in a field of an existing channel.
However, as the Node Bs transmit different ACK/NACK information for the packet data #1 to the UE, the reliability of the ACK/NACK information is not ensured to the UE. Therefore, HARQ for the uplink communication system using the EUDCH must be designed to be different from the HARQ of the HSDPA communication system.